Backflow prevention devices are installed in the main water supply lines leading to industrial and commercial water users, as well as apartment dwellers. These devices arrest any reverse flow of liquid pollutants from those facilities into the main water supply system, in the event of a catastrophic loss or drop in hydraulic pressure in the supply system.
A backflow preventer typically includes a pair of check valves, arranged serially within a valve housing. The housing includes an inlet port for connection to the water system supply line, and an outlet port for connection to the user's incoming water line. When a predetermined water pressure differential exists across the inlet and outlet ports, the force is sufficient to urge the clappers of both check valves into an open position, allowing water to flow freely therethrough. If an insufficient pressure differential exists, the clappers of both check valves are biased into a closed position. Providing the clappers of the closed check valves seal properly, a backflow of liquid, from the outlet port to the inlet port is prevented. A pair of check valves is commonly used for redundancy, in the event one of the valves fails.
There are two primary considerations in the installation and operation of these devices: (1) the need to install, service, and replace the check valves quickly and with relative ease; and, (2) the necessity for the check valves effectively to withstand the considerable pressure which may develop during backflow conditions.
This latter consideration is of particular concern, for example, in systems providing water to chemical or manufacturing plants, where a backflow of toxic liquid into the water supply system could pose a dangerous health hazard.
When water is flowing forwardly through a backflow preventer, the force acting upon the check valve mechanisms in the downstream direction is approximately two percent of the force applied to the valve in the reverse direction, under a backflow condition. Under forward flow conditions, the check valves are easily held or retained in place by conventional fasteners, such as nuts, bolts, screws, or threads. However, under backflow conditions and the much higher attendant forces, these fasteners may allow leakage around the check valve seal, or they may fail entirely. Since even minor leakage of toxins or pollutants could have serious consequences, the need exists for a mounting system which eliminates the possibility of seal compromise.
In prior art backflow preventers employing two check valves, the first, upstream check valve is normally adequately secured by conventional fasteners. This is the case because the base of the first check valve is typically mounted to an attachment structure, such as a flange, a fitting, or a tube, upstream from the base. Forward flow produces only small forces, insufficient to separate the check valve from the upstream attachment structure. And, under backflow conditions, the components which form the seal between the base of the check valve and the attachment structure experience higher compression, thus increasing the reliability of the seal and the integrity of the attachment.
However, a different consideration exists for the second, downstream check valve. In prior art backflow preventers having a line diameter of no greater than 21/2", the base of the second check valve is mounted to an attachment structure, downstream from the base. As would be expected, this arrangement works well during normal, forward flow through the backflow preventer, as the sealing and mechanical attachment components are subjected to compressive forces. However, under the extreme forces cause by backflow pressure, the seal between the second check valve and its downstream attachment structure is challenged. An increase in back pressure applied against the second check valve tends to separate the base of the check valve from the attachment structure. Thus, for backflow preventers in relatively small lines, backflow conditions have exactly the opposite effect upon the integrity of the hydraulic seals between the upstream and downstream check valves and their respective attachment structures.
This potential for leakage increases with larger diameter lines and the greater forces attendant with such lines. Accordingly, prior art backflow preventers for supply lines larger than 21/2" in diameter, employ two inspection ports in the backflow preventer housing. The second inspection port allows the downstream check valve to be mounted to an attachment structure upstream from the base of the valve. This arrangement provides the same enhanced seal reliability for the downstream check, as enjoyed by the upstream check valve during backflow conditions.
Unfortunately, the inclusion of a second inspection port is expensive, increases service time, makes the unit physically longer and provides an additional location where leaks can occur. It would therefore be beneficial to eliminate the need for this second inspection or access port, but retain the sealing advantage afforded by mounting the check valve to an attachment structure upstream from the base of the valve.
Accordingly, it is an object of the present invention to provide an improved backflow prevention device, which effectively resists backflow pressures exerted against it.
It is a further object of the present invention to provide a backflow prevention device having a single access port, permitting quick and easy installation and servicing of both check valves.
It is yet a further object of the present invention to provide a backflow preventer employing upstream and downstream check valve assemblies which are mounted to their attachment structures in the same relation, with respect to the direction of water flow.
It is another object of the present invention to provide check valves assemblies having retaining, sealing, and check valve mechanisms which are structurally and functionally compatible and interchangeable.
It is yet another object of the present invention to provide a method for installing a check valve in a remote, outlet chamber of a backflow preventer housing, through a single access port in an intermediate chamber of the housing.